Building Java Programs Chapter 7 Arrays Copyright (c) Pearson All rights reserved.

2 Can we solve this problem? Consider the following program (input underlined): How many days' temperatures? 7 Day 1's high temp: 45 Day 2's high temp: 44 Day 3's high temp: 39 Day 4's high temp: 48 Day 5's high temp: 37 Day 6's high temp: 46 Day 7's high temp: 53 Average temp = days were above average.

3 Why the problem is hard We need each input value twice: –to compute the average (a cumulative sum) –to count how many were above average We could read each value into a variable... but we: –don't know how many days are needed until the program runs –don't know how many variables to declare We need a way to declare many variables in one step.

4 Arrays array: object that stores many values of the same type. –element: One value in an array. –index: A 0-based integer to access an element from an array. index value element 0element 4element 9

5 Array declaration type [] name = new type [ length ]; –Example: int[] numbers = new int[10]; index value

6 Array declaration, cont. The length can be any integer expression. int x = 2 * 3 + 1; int[] data = new int[x % 5 + 2]; Each element initially gets a "zero-equivalent" value. TypeDefault value int0 double0.0 booleanfalse String or other object null (means, "no object")

7 Accessing elements name [ index ]// access name [ index ] = value ;// modify –Example: numbers[0] = 27; numbers[3] = -6; System.out.println(numbers[0]); if (numbers[3] < 0) { System.out.println("Element 3 is negative."); } index value index value

8 Arrays of other types double[] results = new double[5]; results[2] = 3.4; results[4] = -0.5; boolean[] tests = new boolean[6]; tests[3] = true; index01234 value index valuefalse truefalse

9 Out-of-bounds Legal indexes: between 0 and the array's length - 1. –Reading or writing any index outside this range will throw an ArrayIndexOutOfBoundsException. Example: int[] data = new int[10]; System.out.println(data[0]); // okay System.out.println(data[9]); // okay System.out.println(data[-1]); // exception System.out.println(data[10]); // exception index value

10 Accessing array elements int[] numbers = new int[8]; numbers[1] = 3; numbers[4] = 99; numbers[6] = 2; int x = numbers[1]; numbers[x] = 42; numbers[numbers[6]] = 11; // use numbers[6] as index x numbers x3 index value index value

11 Arrays and for loops It is common to use for loops to access array elements. for (int i = 0; i < 8; i++) { System.out.print(numbers[i] + " "); } System.out.println(); // output: Sometimes we assign each element a value in a loop. for (int i = 0; i < 8; i++) { numbers[i] = 2 * i; } index value

12 The length field An array's length field stores its number of elements. name.length for (int i = 0; i < numbers.length; i++) { System.out.print(numbers[i] + " "); } // output: –It does not use parentheses like a String's.length(). What expressions refer to: –The last element of any array? –The middle element?

13 Weather question Use an array to solve the weather problem: How many days' temperatures? 7 Day 1's high temp: 45 Day 2's high temp: 44 Day 3's high temp: 39 Day 4's high temp: 48 Day 5's high temp: 37 Day 6's high temp: 46 Day 7's high temp: 53 Average temp = days were above average.

14 Weather answer // Reads temperatures from the user, computes average and # days above average. import java.util.*; public class Weather { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("How many days' temperatures? "); int days = console.nextInt(); int[] temps = new int[days]; // array to store days' temperatures int sum = 0; for (int i = 0; i < days; i++) { // read/store each day's temperature System.out.print("Day " + (i + 1) + "'s high temp: "); temps[i] = console.nextInt(); sum += temps[i]; } double average = (double) sum / days; int count = 0; // see if each day is above average for (int i = 0; i < days; i++) { if (temps[i] > average) { count++; } // report results System.out.printf("Average temp = %.1f\n", average); System.out.println(count + " days above average"); }

15 Quick array initialization type [] name = { value, value, … value }; –Example: int[] numbers = {12, 49, -2, 26, 5, 17, -6}; –Useful when you know what the array's elements will be –The compiler figures out the size by counting the values index value

16 "Array mystery" problem traversal: An examination of each element of an array. What element values are stored in the following array? int[] a = {1, 7, 5, 6, 4, 14, 11}; for (int i = 0; i < a.length - 1; i++) { if (a[i] > a[i + 1]) { a[i + 1] = a[i + 1] * 2; } index value index value

17 Limitations of arrays You cannot resize an existing array: int[] a = new int[4]; a.length = 10; // error You cannot compare arrays with == or equals : int[] a1 = {42, -7, 1, 15}; int[] a2 = {42, -7, 1, 15}; if (a1 == a2) {... } // false! if (a1.equals(a2)) {... } // false! An array does not know how to print itself: int[] a1 = {42, -7, 1, 15}; System.out.println(a1); //

18 The Arrays class Class Arrays in package java.util has useful static methods for manipulating arrays: Syntax: Arrays. methodName ( parameters ) Method nameDescription binarySearch( array, value ) returns the index of the given value in a sorted array (or < 0 if not found) copyOf( array, length ) returns a new copy of an array equals( array1, array2 ) returns true if the two arrays contain same elements in the same order fill( array, value ) sets every element to the given value sort( array ) arranges the elements into sorted order toString( array ) returns a string representing the array, such as "[10, 30, -25, 17]"

19 Arrays.toString Arrays.toString accepts an array as a parameter and returns a String representation of its elements. int[] e = {0, 2, 4, 6, 8}; e[1] = e[3] + e[4]; System.out.println("e is " + Arrays.toString(e)); Output: e is [0, 14, 4, 6, 8] –Must import java.util.*;

20 Weather question 2 Modify the weather program to print the following output: How many days' temperatures? 7 Day 1's high temp: 45 Day 2's high temp: 44 Day 3's high temp: 39 Day 4's high temp: 48 Day 5's high temp: 37 Day 6's high temp: 46 Day 7's high temp: 53 Average temp = days were above average. Temperatures: [45, 44, 39, 48, 37, 46, 53] Two coldest days: 37, 39 Two hottest days: 53, 48

21 Weather answer 2 // Reads temperatures from the user, computes average and # days above average. import java.util.*; public class Weather2 { public static void main(String[] args) {... int[] temps = new int[days]; // array to store days' temperatures... (same as Weather program) // report results System.out.printf("Average temp = %.1f\n", average); System.out.println(count + " days above average"); System.out.println("Temperatures: " + Arrays.toString(temps)); Arrays.sort(temps); System.out.println("Two coldest days: " + temps[0] + ", " + temps[1]); System.out.println("Two hottest days: " + temps[temps.length - 1] + ", " + temps[temps.length - 2]); }

Arrays as parameters

23 Swapping values public static void main(String[] args) { int a = 7; int b = 35; // swap a with b? a = b; b = a; System.out.println(a + " " + b); } –What is wrong with this code? What is its output? The red code should be replaced with: int temp = a; a = b; b = temp;

24 Array reversal question Write code that reverses the elements of an array. –For example, if the array initially stores: [11, 42, -5, 27, 0, 89] –Then after your reversal code, it should store: [89, 0, 27, -5, 42, 11] The code should work for an array of any size. Hint: think about swapping various elements...

25 Algorithm idea Swap pairs of elements from the edges; work inwards: index value index value index value index value

26 Flawed algorithm What's wrong with this code? int[] numbers = [11, 42, -5, 27, 0, 89]; // reverse the array for (int i = 0; i < numbers.length; i++) { int temp = numbers[i]; numbers[i] = numbers[numbers.length i]; numbers[numbers.length i] = temp; } The loop goes too far and un-reverses the array! Fixed version: for (int i = 0; i < numbers.length / 2; i++) { int temp = numbers[i]; numbers[i] = numbers[numbers.length i]; numbers[numbers.length i] = temp; }

27 Array reverse question 2 Turn your array reversal code into a reverse method. –Accept the array of integers to reverse as a parameter. int[] numbers = {11, 42, -5, 27, 0, 89}; reverse(numbers); –How do we write methods that accept arrays as parameters? –Will we need to return the new array contents after reversal?...

28 Array parameter (declare) public static type methodName ( type [] name ) { Example: // Returns the average of the given array of numbers. public static double average(int[] numbers) { int sum = 0; for (int i = 0; i < numbers.length; i++) { sum += numbers[i]; } return (double) sum / numbers.length; } –You don't specify the array's length (but you can examine it).

29 Array parameter (call) methodName ( arrayName ); Example: public class MyProgram { public static void main(String[] args) { // figure out the average TA IQ int[] iq = {126, 84, 149, 167, 95}; double avg = average(iq); System.out.println("Average IQ = " + avg); }... –Notice that you don't write the [] when passing the array.

30 Array return (declare) public static type [] methodName ( parameters ) { Example: // Returns a new array with two copies of each value. // Example: [1, 4, 0, 7] -> [1, 1, 4, 4, 0, 0, 7, 7] public static int[] repeat(int[] numbers) { int[] result = new int[2 * numbers.length]; for (int i = 0; i < numbers.length; i++) { result[2 * i] = numbers[i]; result[2 * i + 1] = numbers[i]; } return result; }

31 Array return (call) type [] name = methodName ( parameters ); Example: public class MyProgram { public static void main(String[] args) { int[] iq = {126, 84, 149, 167, 95}; int[] repeated = repeat(iq); System.out.println(Arrays.toString(repeated)); }... Output: [126, 126, 84, 84, 149, 149, 167, 167, 95, 95]

32 Reference semantics 32

33 A swap method? Does the following swap method work? Why or why not? public static void main(String[] args) { int a = 7; int b = 35; // swap a with b? swap(a, b); System.out.println(a + " " + b); } public static void swap(int a, int b) { int temp = a; a = b; b = temp; }

34 Value semantics value semantics: Behavior where values are copied when assigned, passed as parameters, or returned. –All primitive types in Java use value semantics. –When one variable is assigned to another, its value is copied. –Modifying the value of one variable does not affect others. int x = 5; int y = x; // x = 5, y = 5 y = 17; // x = 5, y = 17 x = 8; // x = 8, y = 17

35 Reference semantics (objects) reference semantics: Behavior where variables actually store the address of an object in memory. –When one variable is assigned to another, the object is not copied; both variables refer to the same object. –Modifying the value of one variable will affect others. int[] a1 = {4, 15, 8}; int[] a2 = a1; // refer to same array as a1 a2[0] = 7; System.out.println( Arrays.toString( a1 ) ); // [7, 15, 8] index012 value4158 index012 value7158 a1 a2

36 References and objects Arrays and objects use reference semantics. Why? –efficiency. Copying large objects slows down a program. –sharing. It's useful to share an object's data among methods. DrawingPanel panel1 = new DrawingPanel(80, 50); DrawingPanel panel2 = panel1; // same window panel2.setBackground(Color.CYAN); panel1 panel2

37 Objects as parameters When an object is passed as a parameter, the object is not copied. The parameter refers to the same object. –If the parameter is modified, it will affect the original object. public static void main(String[] args) { DrawingPanel window = new DrawingPanel(80, 50); window.setBackground(Color.YELLOW); example(window); } public static void example(DrawingPanel panel) { panel.setBackground(Color.CYAN);... } panel window

38 Arrays pass by reference Arrays are passed as parameters by reference. –Changes made in the method are also seen by the caller. public static void main(String[] args) { int[] iq = {126, 167, 95}; increase(iq); System.out.println(Arrays.toString(iq)); } public static void increase(int[] a) { for (int i = 0; i < a.length; i++) { a[i] = a[i] * 2; } } –Output: [252, 334, 190] index012 value index012 value iq a

39 Array reverse question 2 Turn your array reversal code into a reverse method. –Accept the array of integers to reverse as a parameter. int[] numbers = {11, 42, -5, 27, 0, 89}; reverse(numbers); Solution: public static void reverse(int[] numbers) { for (int i = 0; i < numbers.length / 2; i++) { int temp = numbers[i]; numbers[i] = numbers[numbers.length i]; numbers[numbers.length i] = temp; } }

40 Array parameter questions Write a method swap that accepts an arrays of integers and two indexes and swaps the elements at those indexes. int[] a1 = {12, 34, 56}; swap(a1, 1, 2); System.out.println(Arrays.toString(a1)); // [12, 56, 34] Write a method swapAll that accepts two arrays of integers as parameters and swaps their entire contents. –Assume that the two arrays are the same length. int[] a1 = {12, 34, 56}; int[] a2 = {20, 50, 80}; swapAll(a1, a2); System.out.println(Arrays.toString(a1)); // [20, 50, 80] System.out.println(Arrays.toString(a2)); // [12, 34, 56]

41 Array parameter answers // Swaps the values at the given two indexes. public static void swap(int[] a, int i, int j) { int temp = a[i]; a[i] = a[j]; a[j] = temp; } // Swaps the entire contents of a1 with those of a2. public static void swapAll(int[] a1, int[] a2) { for (int i = 0; i < a1.length; i++) { int temp = a1[i]; a1[i] = a2[i]; a2[i] = temp; }

42 Array return question Write a method merge that accepts two arrays of integers and returns a new array containing all elements of the first array followed by all elements of the second. int[] a1 = {12, 34, 56}; int[] a2 = {7, 8, 9, 10}; int[] a3 = merge(a1, a2); System.out.println(Arrays.toString(a3)); // [12, 34, 56, 7, 8, 9, 10] Write a method merge3 that merges 3 arrays similarly. int[] a1 = {12, 34, 56}; int[] a2 = {7, 8, 9, 10}; int[] a3 = {444, 222, -1}; int[] a4 = merge3(a1, a2, a3); System.out.println(Arrays.toString(a4)); // [12, 34, 56, 7, 8, 9, 10, 444, 222, -1]

43 Array return answer 1 // Returns a new array containing all elements of a1 // followed by all elements of a2. public static int[] merge(int[] a1, int[] a2) { int[] result = new int[a1.length + a2.length]; for (int i = 0; i < a1.length; i++) { result[i] = a1[i]; } for (int i = 0; i < a2.length; i++) { result[a1.length + i] = a2[i]; } return result; }

44 Array return answer 2 // Returns a new array containing all elements of a1,a2,a3. public static int[] merge3(int[] a1, int[] a2, int[] a3) { int[] a4 = new int[a1.length + a2.length + a3.length]; for (int i = 0; i < a1.length; i++) { a4[i] = a1[i]; } for (int i = 0; i < a2.length; i++) { a4[a1.length + i] = a2[i]; } for (int i = 0; i < a3.length; i++) { a4[a1.length + a2.length + i] = a3[i]; } return a4; } // Shorter version that calls merge. public static int[] merge3(int[] a1, int[] a2, int[] a3) { return merge(merge(a1, a2), a3); }

Arrays for tallying

46 A multi-counter problem Problem: Write a method mostFrequentDigit that returns the digit value that occurs most frequently in a number. –Example: The number contains: one 0, two 2s, four 6’s, one 7, and one 9. mostFrequentDigit( ) returns 6. –If there is a tie, return the digit with the lower value. mostFrequentDigit( ) returns 3.

47 A multi-counter problem We could declare 10 counter variables... int counter0, counter1, counter2, counter3, counter4, counter5, counter6, counter7, counter8, counter9; But a better solution is to use an array of size 10. –The element at index i will store the counter for digit value i. –Example for : –How do we build such an array? And how does it help? index value

48 Creating an array of tallies // assume n = int[] counts = new int[10]; while (n > 0) { // pluck off a digit and add to proper counter int digit = n % 10; counts[digit]++; n = n / 10; } index value

49 Tally solution // Returns the digit value that occurs most frequently in n. // Breaks ties by choosing the smaller value. public static int mostFrequentDigit(int n) { int[] counts = new int[10]; while (n > 0) { int digit = n % 10; // pluck off a digit and tally it counts[digit]++; n = n / 10; } // find the most frequently occurring digit int bestIndex = 0; for (int i = 1; i < counts.length; i++) { if (counts[i] > counts[bestIndex]) { bestIndex = i; } return bestIndex; }

50 Array histogram question Given a file of integer exam scores, such as: 82, 85, 90, 70, 98, 82, 66, 70, 63, 83 88, 78, 70, 81, 88, 91, 75, 70, 72, 88 84, 88, 75, 70, 78, 86, 91, 98, 86, 88 Write a program that will print a histogram of stars indicating the number of students who earned each unique exam score. 85: ***** 86: ************ 87: *** 88: * 91: ****

51 Array histogram answer // Reads a file of test scores and shows a histogram of score distribution. import java.io.*; import java.util.*; public class Histogram { public static void main(String[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("midterm.txt")); int[] counts = new int[101]; // counters of test scores while (input.hasNextInt()) { // read file into counts array int score = input.nextInt(); counts[score]++; // if score is 87, then counts[87]++ } for (int i = 0; i < counts.length; i++) { // print star histogram if (counts[i] > 0) { System.out.print(i + ": "); for (int j = 0; j < counts[i]; j++) { System.out.print("*"); } System.out.println(); }

52 Section attendance question Read a file of section attendance (see next slide): yynyyynayayynyyyayanyyyaynayyayyanayyyanyayna ayyanyyyyayanaayyanayyyananayayaynyayayynynya yyayaynyyayyanynnyyyayyanayaynannnyyayyayayny And produce the following output: Section 1 Student points: [20, 17, 19, 16, 13] Student grades: [100.0, 85.0, 95.0, 80.0, 65.0] Section 2 Student points: [17, 20, 16, 16, 10] Student grades: [85.0, 100.0, 80.0, 80.0, 50.0] Section 3 Student points: [17, 18, 17, 20, 16] Student grades: [85.0, 90.0, 85.0, 100.0, 80.0] Students earn 3 points for each section attended up to 20.

53 –Each line represents a section. –A line consists of 9 weeks' worth of data. Each week has 5 characters because there are 5 students. –Within each week, each character represents one student. a means the student was absent(+0 points) n means they attended but didn't do the problems(+2 points) y means they attended and did the problems(+3 points) Section input file yynyyynayayynyyyayanyyyaynayyayyanayyyanyayna ayyanyyyyayanaayyanayyyananayayaynyayayynynya yyayaynyyayyanynnyyyayyanayaynannnyyayyayayny week student section 1 section 2 section 3

54 Section attendance answer import java.io.*; import java.util.*; public class Sections { public static void main(String[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("sections.txt")); int section = 1; while (input.hasNextLine()) { String line = input.nextLine(); // process one section int[] points = new int[5]; for (int i = 0; i < line.length(); i++) { int student = i % 5; int earned = 0; if (line.charAt(i) == 'y') { // c == 'y' or 'n' or 'a' earned = 3; } else if (line.charAt(i) == 'n') { earned = 2; } points[student] = Math.min(20, points[student] + earned); } double[] grades = new double[5]; for (int i = 0; i < points.length; i++) { grades[i] = * points[i] / 20.0; } System.out.println("Section " + section); System.out.println("Student points: " + Arrays.toString(points)); System.out.println("Student grades: " + Arrays.toString(grades)); System.out.println(); section++; }

55 Data transformations In many problems we transform data between forms. –Example: digits  count of each digit  most frequent digit –Often each transformation is computed/stored as an array. –For structure, a transformation is often put in its own method. Sometimes we map between data and array indexes. –by position(store the i th value we read at index i ) –tally(if input value is i, store it at array index i ) –explicit mapping(count 'J' at index 0, count 'X' at index 1) Exercise: Modify our Sections program to use static methods that use arrays as parameters and returns.

56 Array param/return answer // This program reads a file representing which students attended // which discussion sections and produces output of the students' // section attendance and scores. import java.io.*; import java.util.*; public class Sections2 { public static void main(String[] args) throws FileNotFoundException { Scanner input = new Scanner(new File("sections.txt")); int section = 1; while (input.hasNextLine()) { // process one section String line = input.nextLine(); int[] points = countPoints(line); double[] grades = computeGrades(points); results(section, points, grades); section++; } // Produces all output about a particular section. public static void results(int section, int[] points, double[] grades) { System.out.println("Section " + section); System.out.println("Student scores: " + Arrays.toString(points)); System.out.println("Student grades: " + Arrays.toString(grades)); System.out.println(); }...

57 Array param/return answer... // Computes the points earned for each student for a particular section. public static int[] countPoints(String line) { int[] points = new int[5]; for (int i = 0; i < line.length(); i++) { int student = i % 5; int earned = 0; if (line.charAt(i) == 'y') { // c == 'y' or c == 'n' earned = 3; } else if (line.charAt(i) == 'n') { earned = 2; } points[student] = Math.min(20, points[student] + earned); } return points; } // Computes the percentage for each student for a particular section. public static double[] computeGrades(int[] points) { double[] grades = new double[5]; for (int i = 0; i < points.length; i++) { grades[i] = * points[i] / 20.0; } return grades; }